Aluminum production method



Patented Apr. 24, 1962 3,031,294 ALUMINUM PRUDUCTION METHOD Alan W.Searcy, Grinda, Calif., and David J. Meschi, Highland, Ind., assignorsto the United States of America as represented by the United StatesAtomic Energy Commission No Drawing. Filed June 15, 1959, Ser. No.820,565 1 Claim. (Cl. 75-68) The present invention refers to an aluminumproduc tion method and, more particularly, to a method for producingaluminum by the reduction of aluminum varbide under controlledconditions.

The production of metallic aluminum has been accomplished, with varyingdegrees of success, by a variety of processes involving the reduction ofan aluminum compound of higher valence to its lowest valence or metallicstate. One of the less successful of these methods attempted by earlyresearchers comprised the preparation of aluminum carbide by the carbonreduction of aluminum oxide and the subsequent distillation-reduction ofthe carbide to produce aluminum. Although it was possible to obtainmetallic aluminum in this manner, the resulting product was of extremelypoor quality being highly contaminated with carbon, aluminum carbide,aluminum oxide and other aluminum-carbon compounds; consequently,experiments along this line were largely abandoned. The presentinventors, however, have succeeded in discovering those conditions underwhich aluminum carbide reduction may be performed to obtain an aluminummetal of much greater purity than was previously obtainable by thismethod.

In addition to advancing the state of the art by providing means for thesuccessful reduction of aluminum carbide to aluminum metal, thisinvention also has considerable advantages from an economic standpointin that this process requires considerably less energy than theelectrolytic reduction of aluminum oxide which latter process is thatemployed by the majority of commercial aluminum metal manufacturers atpresent. In the electrolytic process 280 kilocalories are required toproduce 2 mols of aluminum gas from aluminum oxide Whereas in thepresent invention only 96 kilocalories are required to produce the sameamount of aluminum gas from aluminum carbide. This invention is alsoimportant in that it may be efficiently performed in geographical areaswhere large quantities of electrical energy are unduly expensive.

Accordingly, it is an object of this invention to provide anadvantageous method for the production of aluminum metal by the thermalreduction of aluminum carbide.

It is another object of this invention to provide a method for reducingaluminum carbide whereby the resulting aluminum product has a puritygreater than that obtained by previous aluminum carbide reductionmethods.

It is still another object of this invention to provide a process forthe production of metallic aluminum which requires a relatively smallamount of energy in comparison with conventional aluminum productiontechniques.

It is a still further object of this invention to provide a method forthe production of aluminum metal by the reduction of aluminum carbideunder closely controlled conditions so as to minimize carboncontamination of the finished product.

Considering now the particulars of the invention, it has been known thatmetallic aluminum could be obtained by the thermal distillation ofaluminum carbide according to the reaction:

This method as heretofore performed, however, suffered from thelimitation that the aluminum metal so produced contained a relativelyhigh proportion of undesirable contaminants thus rendering it unsuitablefor further usage without extensive purification. Therefore, it hasgenerally been considered impractical to employ this method incommercial operation. The present inventors, however, have investigatedthe method and successfully determined those factors which contributedto the poor quality of the aluminum. By the elimination, therefore, ofthese undesirable factors and by the selection ofcarefully determinedand unique reaction conditions, the method of the present invention wasdeveloped.

Among the more objectionable features which were found to be inherent inthe previously practiced method, and which are therefore eliminated fromthe method of the present invention, was the presence of moisture in thereaction system, the moisture being supplied either by a wet startingmaterial or, more probably, by the atmosphere. The action of water onaluminum carbide causes hydrolysis of the latter compound withsubsequent formation of aluminum hydroxide, methane and other low weighthydrocarbons. Due to their volatility the hydrocarbons escape easily andpresent no contamination problem. Aluminum hydroxide, however, willdecompose on heating to release water, and thus effectively producealuminum oxide. This oxide then reacts with the aluminum carbidestarting material to give free aluminum and carbon monoxide.

On first consideration it would appear that this chain of reactionsproducing the desired end product, free aluminum, and a gaseousby-product carbon monoxide would be beneficial and in no way deleteriousto a process designed for the production of high quality aluminum metal.Such is not the case, however, as studies have shown that this vaporizedaluminum, as well as aluminum produced by the primary reaction of thismethod, is acted upon by the carbon monoxide at or before that surfaceof the reaction vessel at which the aluminum metal becomes condensed.Here the combination of these compounds produces both aluminum oxide anda variety of condensed carbon and aluminum-carbon products, the presenceof which is considered undesirable in finished aluminum. It is onefeature of the present invention therefore to prevent the formation ofthese contaminants by providing for the exclusion of moisture from theprocess. The manner in which the exclusion of moisture and otherundesirable substances may be accomplished will be discussed in moredetail hereinafter.

Considering further the distinguishing features of the presentinvention, investigations as to the inferiority of prior aluminumcarbide reduction methods have also shown that it is undesirable toallow oxygen in the reaction system while the reaction mixture is hot,or at temperatures at which the aluminum carbide dissociation can beeffected. The end result of oxygen contamination is the same as that dueto moisture; that is the ultimate contaminants produced by each of theseinterfering sub stances are identical. In brief, the sequence ofreactions initiated by oxygen are as follows: Aluminum oxide and carbonmonoxide are formed by the interaction of oxygen and aluminum carbide.The aluminum oxide in turn reacts with aluminum carbide to produce freealuminum and carbon monoxide; as has been previously pointed out, thislatter material has been shown to combine with free vaporized aluminumat or before the condensing surface whereupon there is again formedaluminum oxide as well as various carbon products.

It is an essential feature of the present invention therefore to performthe reduction of aluminum carbide under such conditions as to preventoxygen from combining with the aluminum carbide dissociation products.The manner in which this is accomplished will be subsequently described.

Still another factor to be taken into consideration when thermallyreducing aluminum carbide is the undesired effect on the final aluminumproduct due to the presence of nitrogen and the consequent interactionof this substance with aluminum carbide. It has been discovered that asnitrogen becomes available to vaporized aluminum carbide, a monovalentaluminum cyanide compound, AlCN, is formed. This compound per se is notfound among the contaminants of the finished aluminum as it is not knownto exist in the solid state; rather, on cooling, it decomposes to yieldaluminum nitride and carbon. If during the cooling operation, however,aluminum cyanide comes into contact with gaseous aluminum, both thenitride and carbide of aluminum are likely to be formed. In either eventthe finished aluminum will be less pure, and consequently lesscommercially acceptable, in proportion to the amount of nitrogen whichis permitted. Accordingly, it is another distinguishing feature of thepresent invention to provide for the reduction of aluminum carbide toaluminum metal in a nitrogen free atmosphere.

With respect now to the manner in which the afore mentioned features andconditions of the invention may be realized, both the aluminum carbidestarting material and the gaseous atmosphere in which this material isto be reduced must be so chosen as to substantially exclude all tracesof moisture, oxygen and nitrogen containing compounds.

Considering first aluminum carbide, care must be taken to select thatmaterial which contains little or none of the objectionable substanceslisted above. echnical grade aluminum carbide prepared by the reductionof aluminum oxide over an excess of carbon is quite satisfactory forthis purpose as a high carbon atmosphere supports the reduction ofvirtually all aluminum nitride and oxide materials originally present inthe aluminum oxide. Moisture, of course, will be driven out by the heatof this process and careful handling will insure the 'maintenance of thematerial in a dry state.

Control of the atmosphere in order to prevent moisture, oxygen andnitrogen from coming into contact with dissociating aluminum carbide,may be achieved by one of two methods. The first of these comprisesconducting this process under a high vacuum whereby these substances areautomatically removed by evacuation in proportion to the degree ofvacuum achieved. Inasmuch as the partial pressure of aluminum is fairlyindependent of the total surrounding pressure, the vacuum pressureselected here is not critical to the thermal dissociation of aluminumcarbide within the temperature range recommended below but is chosen forthe express purpose of obtaining a non-contaminated atmosphere. For thisreason it is desirable to employ as high a vacuum as is possiblecommensurate with efiicient and attainable operating conditions. Apressure of about atmospheres is suggested as satisfactory but shouldnot be taken to be a limiting condition for this process.

An alternate method by which to prevent contamination of dissociatingaluminum carbide comprises conducting this reduction process in thepresence of a carefully dried, oxygen and nitrogen free inert gas suchas helium or argon. In this manner, as with the above described vacuumtechnique, it is possible to selectively control the atmosphere withwhich the reactants are to come into contact and thus successfullyeliminate those materials which have been found to adversely affect thequality of the finished aluminum. This atmospheric control is thereforean important aspect of the present invention.

In addition to the aforementioned precautions for the exclusion ofcertain substances from the reactants and surrounding atmosphere, itwill be found that temperature is still another factor which isessential to performing the reduction of aluminum carbide in order toobtain a high purity aluminum product. The carryover of carboncontaminants into the distilled aluminum is increased at highertemperatures due to the formation of the unstable aluminum carbide, Al CThis compound, which is known to exist in the gaseous state only,becomes entrained by the aluminum vapor produced by Al C thermaldissociation and on cooling, further reacts with this aluminum to giveAl C again; this material in turn becomes physically mixed with thefinal aluminum metal product, thereby increasing the amount ofcontamination. Studies have shown that the formation of this unstablecarbide is enhanced by increased temperature and, conversely, isminimized by lower heats. Therefore, the process of the presentinvention is best conducted in that temperature range which issufiicient to provide energy for the dissociation of Al C but not sogreat as to cause excessive formation of Al C The range of about 1600 K.to about 2300 K. then has been found to satisfy the above requirement,and accordingly is prescribed as a necessary condition to practicingthis invention.

In summary, the present invention may be described as an improved methodfor the production of aluminum metal by the dissociation of aluminumcarbide under selected conditions which have been shown to effectivelyreduce the contamination of the finished metal, especially contaminationby carbon compounds, which method comprises thermally reducing analuminum carbide material substantially free of moisture, oxygen andnitrogen containing compounds in an atmosphere which is so controlled asto exclude these same objectionable substances and at a temperaturewithin the range of about 1600 K. to about 2300" K.

The following example is offered as one illustration of the method andthis invention in which the advantageous aspects thereof aredemonstrated.

Example Into a covered graphite crucible having a central opening in thelid thereof, there was put a quantity of aluminum carbide which had beenproduced in such a manner as to contain substantially no aluminum oxide,aluminum nitride or other oxygen or nitrogen containing materials, andwhich also was carefully dried so as to insure the absence of moisture.This crucible was next placed in a dry vacuum chamber which was thenclosed and evacuated to an internal pressure of about 10 atmospheres andmaintained at this pressure for the duration of the dissociationreaction. Radio-frequency electromagnetic induction was used as the heatsource to bring the crucible contents to a temperature within the rangeof about 1600 to 2300 K. Under these conditions aluminum carbide becamedissociated to form aluminum in the gaseous state and solid particles ofcarbon. As the dissociation proceeded the vaporized aluminum flowed fromthe crucible in a unidirectional stream and upon contacting a coolerportion of the reaction equipment condensed finally to yield solidifiedaluminum metal.

Heating of the crucible was continued until all aluminum carbide wasconverted to free aluminum and carbon. Examination of the aluminumrecovered at the conclusion of this reaction process showed that underthese carefully controlled conditions a minimum of contamination,particularly that due to carbon, was allowed. By spectroscopicprocedures, the carbon content was shown to be considerably less than1%, and by established procedures, removal of impurities could be easilyaccomplished.

What is claimed is:

A process for the production of aluminum metal comprising the productionof aluminum carbide by reduction of a mixture consisting of aluminumoxide in the presence References Cited in the file of this patent of anexcess of carbon, and the thermal reduction of said UNITED STATESPATENTS aluminum carb1de within a vacuum of about 10 atmospheres, saidthermal reduction being carried out at a 2,776,884 Grunert 1957temperature substantially Within the range 1600" K. to 5 FOREIGN PATENTS2300 K, whereby aluminum is extracted in the gaseous state, saidreaction being carried out in the absence of 482,150 Great Britain Mar.24, 1938 nitrogen.

23,315 Great Britain 1909

